1. Field of the Invention
This invention relates to mixers in general and more particularly to a double balanced mixer that minimizes inter-modulation products, has a high third order intercept point (IP3) and that has an intermediate frequency (IF) response down to direct current (DC) levels.
2. Description of the Prior Art
A mixer circuit converts an RF signal to an intermediate frequency (IF) signal which is the difference of the RF signal and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates intermodulation products due to the non-linearity response. Third order intermodulation products are close in frequency to the fundamental IF frequencies and therefore are difficult to remove by filtering. Third-order intermodulation distortion is a measure of the third-order products generated by a second input signal arriving at the input of a mixer along with the desired signal.
One technique to measure the suppression capability of a mixer is the “third-order intercept” approach. The third-order intercept point is a theoretical point on the RF input versus IF output curve where the desired output signal and third-order products become equal in amplitude as RF input is raised. The high end of the dynamic range of the mixer is defined as the maximum received signal power at which the mixer is designed to be used and is designated the 1 dB compression point or the input signal power level at which the power level of a third order product equals the power level of a fundamental IF signal is called the third order intercept point (IP3). A mixer with a higher IP3 value will have better performance. A mixer is usually specified in terms of input IP3. Output third order intercept point is the difference between input IP3 and conversion loss. Higher conversion losses result in lower output IP3.
Conversion loss is a measure of the efficiency of the mixer in providing frequency translation between the input RF signal and the output IF signal. Conversion loss of a mixer is equal to the ratio of the IF output to the RF input level.
Mixers are typically designed with one of three topologies: single ended, balanced, and double balanced. The double balanced mixers are capable of isolating both the RF signal and the local oscillator LO voltages from the output and thus allow overlap of the RF and IF frequency bandwidths. Several prior art mixer circuits are well known. One mixer design uses a schottky diode quad or ring circuit that uses four diodes with all of the diodes pointed in the same direction. Another mixer circuit is called a star circuit, which uses two diodes pointing toward the central node and two diodes pointing away from the central node. Schottky diode mixers approaching 30 dBm are difficult to tune and are expensive. Diode mixers also require large LO signal levels to obtain a high IP3 which is not practical in many systems.
Another type of mixer uses field effect transistors (FET) as the mixing element instead of a schottky diode. Mixers fabricated using FET's can achieve a higher value of IP3. Unfortunately, mixers using FET's have several other disadvantages such as higher conversion losses and a higher noise figure (conversion losses at 1 dB).
Today's communication receivers operate under multiple frequency carrier environments. Some of these multiple frequency carriers are wanted and some are unwanted. When these frequency carriers are present in a receiver or transmitter simultaneously, they generate inter-modulation products that are unwanted. The level of the inter-modulation products is dictated by the linearity of the mixer in the receiver. The degree of non-linearity of the mixer is measured by IP3. High IP3 mixers are desired for receivers and transmitters operating in multiple carrier environments to minimize third order inter-modulation products.
Mixers with an IF response down to DC levels are particularly useful in low IF mixers, phase detectors and in-phase quadrature (IQ) modulators. IQ modulation is an efficient way to transfer information. IF response down to DC levels is extremely important for mixers used in IQ modulators.
While FET mixers have been used, a current unmet need exists for an improved double balanced FET mixer that has a high third order intercept point and IF response down to DC levels. Another current unmet need is for an improved FET mixer for IQ modulator applications that has an IF response down to DC levels and that can be manufactured at low cost.